1. Field of the Invention
The present invention relates to a drive power transmission control device for a front-and-rear wheel drive vehicle.
2. Discussion of the Related Art
Heretofore, as drive power transmission control devices of this kind, there has been known one which is described in Japanese unexamined, published patent application No. 2002-340054. The known control device is provided with an electromagnetic type drive power transmission device which contains in a casing a clutch mechanism for transmitting the drive power transmitted from an engine to either pair of front and rear wheels, to the remaining pair of the wheels and an electromagnetic coil for controlling the torque transmitted by the clutch mechanism. The control device is further provided with an electronic control device for controlling the electric current flow to the electromagnetic coil of the electromagnetic type drive power transmission device.
More specifically, the drive power transmission control device is incorporated in a four-wheel drive vehicle M, as shown in FIG. 9, and is provided with an electromagnetic type drive power transmission device 2 capable of varying the transmission torque which connects right and left front wheels Wfr, Wfl driven by an engine 1 to right and left rear wheels Wrr, Wrl and an ECU (Electronic Control Unit) 3 as the electronic control device for controlling the transmission torque of the electromagnetic type drive power transmission device 2. The drive power from the engine 1 is transmitted to a transfer 5 through a transaxle 4 incorporating a transmission (not shown) therein, and the drive power distributed to the front wheel side is transmitted to the right and left front wheels Wfr, Wfl through a front differential gear (not shown), while the drive power distributed to the rear wheel side is transmitted to the right and left rear wheels Wrr, Wrl through a first propeller shaft 6a, the electromagnetic type drive power transmission device 2, a second propeller shaft 6b and a rear differential gear 7.
The electromagnetic type drive power transmission device 2 contains in the casing 2a a clutch mechanism 2b for transmitting the drive power transmitted from the engine 1 to the pair of the front wheels Wfr, Wfl, to the other pair of the rear wheels Wrr, Wrl and the electromagnetic coil 2c for controlling the transmission torque of the clutch mechanism 2b. The ECU 3 is provided for controlling the electric current flow to the coil 2c and is arranged in a space for driver's legs or in an engine room. The ECU 3 and the coil 2c are connected by means of a harness 8.
In the foregoing drive power transmission control device, the electromagnetic type drive power control device 2 and the ECU 3 are separated, wherein the electromagnetic type drive power transmission device 2 is of the nature involving nonuniformity in mechanical quality, while the ECU 3 is of the nature involving nonuniformity in electrical quality. Therefore, it has been laborious to adjust these kinds of nonuniformity as a whole. Further, the electromagnetic type drive power transmission device 2 and the ECU 3 are individually arranged at respective places remote from each other, and this has given rise to another problem that a long harness 8 is required thereby resulting in an increase of the vehicle weight.
Further, a drive power transmission device of this kind usually incorporates therein an electromagnetic clutch mechanism of multiple wet plate type and brings clutch plates into friction engagement by controlling the application of electric current to an electromagnetic coil incorporated in the clutch mechanism 2b, so that the drive power of an engine can be transmitted to rear wheels through propeller shafts, a rear differential gear and a pair of right and left rear axles. The friction engagement force of the clutch plates depends on the magnitude or value of electric current applied to the electromagnetic coil, and the drive power transmitted to the rear wheels increases as the friction engagement force becomes large. That is, in a four-wheel drive vehicle called as “Stand-by Type”, by controlling the friction engagement force of the clutch plates, the drive power transmission device selects either a four-wheel drive state or a two-wheel drive state and controls the distribution ratio of the drive power between the front wheels and the rear wheels in the four-wheel drive state.
As the drive power distribution to the rear wheels increases, the friction engagement force of the clutch plates becomes large, whereby friction heat generated on the clutch plates causes the temperature of lubricant oil filled in the drive power transmission device to rise. Lubricant oil has the nature to lower its viscosity with increase in its temperature and hence, to make the transmitted drive power small. To transmit a desired drive power in this situation, it is required to make the friction engagement force of the clutch plates larger. However, making the friction engagement force of the clutch plates larger causes the oil temperature to rise higher, which would leads to failure of the drive power transmission device. For this reason, as described in Japanese unexamined, published patent applications No. 2003-136989 and No. 2003-136990, there have been proposed drive power transmission devices, wherein a temperature sensor is provided for detecting the temperature of the lubricant oil and wherein, when the oil temperature exceeds a predetermined value, the distribution rate of the drive power is lowered so that the friction engagement force of the clutch plates is weakened thereby to lower the oil temperature.
However, in these known devices, an ECU is mounted usually on a place which is not liable to be influenced by water and dust and therefore, is located remote from the drive power transmission device. Therefore, the temperature sensor has to be attached inside a housing of the drive power transmission device, and leads (i.e., cable or harness) extending from output terminals of the temperature sensor have to be taken out from the housing. Further, sealing means has to be interposed between the leads and the housing to prevent water and dust from entering the inside of the housing. Consequently, the prior art drive power transmission devices give rise to problems that they requires laborious work in attaching the temperature sensor and incur a substantial increase in the manufacturing cost.
To obviate these drawbacks, one may conceive of a construction which arranges the aforementioned temperature sensor not inside the drive power transmission device but at a portion which is somewhat distant from the drive power transmission device, to detect the temperature of the same and which utilizes the detected temperature and the electric current applied to the electromagnetic coil in calculating the temperature of the lubricant oil. However, where the construction is taken, additional measures for, e.g., attaching a case with a sealing structure to the vehicle body and for attaching the temperature sensor inside the case, have to be taken to protect the temperature sensor from water or dust. This also makes it laborious to attach the temperature sensor.
Further, where a heat resistance element of the character which makes its resistance vary in dependence on temperature is used as the temperature sensor, it is possible that a precise resistance value (i.e., temperature) cannot be detected under the influence of resistance components included in the leads themselves. It is also possible that a precise temperature cannot be detected under the influence of noise which would be generated in dependence on the length or wiring arrangement.
Where the temperature sensor is connected to the ECU by means of leads, it has been a practice to use connectors, for which water-proof capability is essential. In the case of the connection using connectors, it is possible that a precise resistance value cannot be detected under the influence of friction resistances at connector pins. In addition, incomplete joining of the connectors would make it impossible to detect a precise resistance value or the temperature.